Gravure cylinders comprise of a base cylinder, which is usually made of steel or aluminum (1, FIG. 1), a “soft” copper layer (2, FIGS. 1 and 2, FIG. 2) usually 10 μm thick, a “hard” copper layer usually 0.5 to 1 mm thick (3, FIGS. 1 and 3, FIG. 2) and a protection layer, which is usually a chromium layer typically 6 to 8 μm thick (4, FIGS. 1 and 4, FIG. 2).
The “hard” copper layer is electroplated on the base of the cylinder and forms the surface which is engraved or etched either by chemical or electromechanical (diamond) or electronic (laser) method with the pattern which will be printed (transferred) on the packaging material (paper, plastic film, aluminum foil, etc.). The copper is the dominant surface used for engraving, because it is easy to engrave. The chromium layer on the engraved cylinder protects the surface of the cylinder from the pressure exerted by the doctor blade on the printing cylinder during the printing process (transfer of ink onto the packaging material).
The cylinder body is usually made of steel which satisfies the requirements for precision and small deflection required in the printing process. Alternatively for the printing industry, the cylinder body can be manufactured from a light weight metal like aluminum or an aluminum alloy. Aluminum has specific weight of about 2700 kg/m3, while steel has a specific weight of about 7800 kg/m3. Using aluminum as the cylinder base results in a lighter rotogravure cylinder (by about one third) which means significant reduced transportation costs and safer handling during production phases.
However, aluminum is an electrochemically passive material and it is quite challenging to electro-copper plate it. This has limited the use of aluminium for the base of the cylinder. To the extent that aluminium is used, it requires a plurality of process steps so as to obtain a suitable copper surface for the aluminium body.
One method for the manufacture of rotogravure cylinders comprising an aluminium base, a copper surface and a chrome protection layer is known from WO2011/073695A2. The copper surface is created in a process that comprises several steps.
In a first step, the surface roughness of the underlying cylinder is increased by a mechanical means, such as sand paper, sandblasting. Thereafter, a copper coating of 10-50 μm thickness is deposited in a thermal spraying process. The copper coating is considered to be the substrate for subsequent electroplating. Another surface treatment with sandpaper is then carried out.
In the subsequent step, a pre-copper plating step is carried out, wherein a layer of copper of about 100-300 μm is plated. The copper is plated without hardener, resulting in a Vickers hardness of 100-120 HV.
This step is followed by another copper plating step, using a bath that includes a hardener, so as to obtain a copper engraving layer with preferably a Vickers hardness of 200-240 HV. Such Vickers hardness is known to be optimal for engraving; at lower values, the engraved cell pattern loses definition. In addition, if the hardness exceeds 240 HV, the lifetime of the diamond styli often used to engrave the cylinders during electronic engraving may be reduced. The copper engraving layer of WO2011/073695 is deposited in a thickness of about 200 μm. Finally, a polishing step is carried out to achieve a predetermined surface roughness, suitably in the range of 0.03-0.07 mm.
According to this method, the very hard copper engraving layer is supported with a stack that is less hard. As is well-known, the Vickers hardness of aluminium or an aluminium alloy is relatively low; a medium strength aluminium alloy such as aluminium alloy 6082 is known to have a Vickers hardness of 35 HV. The copper support comprising the copper adhesion layer and particularly the pre-plated layer therewith has an intermediate hardness between the aluminium base and the hard copper layer.
Moreover, in accordance with this method, about half of the at least 0.5 mm copper layer is present as support. This layer thickness is needed, so as to obtain an appropriately homogeneous layer microstructure on top of which the hard copper can be grown.
In further investigations on the cylinders manufactured in accordance with WO2011/073695A2 it was however observed that the reliability of the cylinders was less than desired. Particularly, about 1-5% of the cylinders turned out defect relatively quickly after use by the customer. However, the defects came irregularly, in an unpredictable manner. Such defect clearly resulted in a need for replacement of the defect cylinders, which is undesired.
In another non-prepublished application in the name of the applicant (application number EP12187941.5) the copper support consists of a single layer, and nevertheless matches the difference in properties between the base and the copper engraving layer with a high hardness. The copper particles are suitably deposited in a high velocity spraying process which results in liberation of a significant amount of energy in the form of heat. This heat will warm up the particles so as to melt at least partially. This invention results in light weight gravure cylinders without the drawbacks of previous inventions.
The inherent disadvantage of the prior art is the use of electrolytic baths that constitutes a hazard to the environment and to humans.